Quasi-random pneumatic vibration facility and automatic frequency modulating system therefor

ABSTRACT

A low-cost, multi-axis, quasi-random vibration system includes pneumatically driven vibrators coupled to a resonating, self-attenuating shaker structure to achieve a frequency spectrum and acceleration-level control of a broadband quasi-random vibration output in the frequency range, for example, from 40 Hz to 2 kHz for vibration testing of equipment. The shaker structure consists of flexible interconnected structures whose primary or driving structure acts to mechanically process the vibratory input of the coupled vibrators and whose supporting or driven structure supplies a further vibrational processing to supply the test item with multi-axis multimodal input. Automatic control and pseudo-random modulation of air pressure of the pneumatic vibrators provide closed-loop broadband acceleration-spectrum control and spectrum smearing to enhance frequency content and to prevent the shaker from locking onto any particular vibration frequency, especially a natural frequency of the shaker. Control is achieved in and about three orthogonal axes simultaneously, thus affording a realistic simulation of operational environments. The vibrators&#39; vibration frequency is modulated by means of a variable-area orifice in the pneumatic line located between the air supply and the pneumatic vibrators. A pseudo-random change in the orifice area is made automatically, for example, every 2 to 3 seconds, by a preprogrammed microprocessor-controlled flow control mechanism.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to vibration test equipment employingpneumatic vibrators and incorporating automatic frequency modulation,e.g., for proof of workmanship, for operational screening, and forengineering development tests.

As used herein, the terms "quasi-random" and "pseudo-random" are definedas follows. "Quasi-random" vibration can be described as a line spectrumwith equally spaced lines, e.g., harmonics, whose fundamental frequencyvaries randomly with time within a restricted frequency range, e.g.,vibrator frequency excursion during modulation, which, in turn, causes arandom fluctuation in the amplitudes, (e.g., accelerations) at thespectral lines. The fundamental frequency fluctuates sufficiently toproduce an essentially continuous spectrum when averaged over a longenough time interval. By "pseudo-random", it is meant that there is amathematical method or algorithm for selecting a sequence of numbers,e.g., for use in modulating the driving means for the vibrators."Pseudo" means that the randomness is not purely random because thesequence results from predetermined calculations.

Also as used herein, the terms "multi-degree-of-freedom", "spectrum" and"multi-modal" are defined as follows. "Multi-degree-of-freedom" is usedto define the ability of structure to translate and rotate in severaldirections simultaneously within given bounds. "Spectrum", e.g., as in"intense vibration spectrum", means the cumulative time history of thevibration (e.g., in G² /Hz) as related to the frequency associated witha particular level of vibration. "Multi-modal" means the simultaneousoccurrence of many structural vibrational modes, or dynamicdisplacements.

2. Description of the Prior Art and Background Considerations

In the prior art, vibration screening of equipment, e.g., airborne radarunits, infrared sensors and missiles, was accomplished by single-axismechanical vibration apparatus working with fundamental frequency ofexcitation and uncontrolled harmonics. Alternatively, electrodynamicshakers and control systems were employed for single-axis or, in groups,multi-axis testing. Such systems are very expensive, and multi-axisconfigurations present problems as to the coherence of accelerationinputs.

The use of multiple pneumatic vibrators for the simulation of randomvibration was first suggested by General Dynamics. Corporation in apaper in Shock & Vibration Bulletin, No. 46, Part 3, August 1976, pp.1-14. This paper describes a missile test in which nine pneumaticvibrators are attached directly to a freely suspended missile. Anapproximation of measured in-flight random vibration was obtained.Frequency spectrum and acceleration level were determined by the numberand size of the attached vibrators and the mean air pressure. The supplypressure was modulated in a periodic fashion to prevent locking on thefirst bending mode of the missile structure and to fill in the frequencyspectrum. In a panel discussion reported in the Journal of EnvironmentalSciences, November/December 1976, pp. 32-38, Westinghouse ElectricCorporation discloses development of a pneumatic vibrator system fortesting avionics equipment. Pneumatic vibrators are said to be attacheddirectly to rigid vibration fixtures to achieve a two-axis excitation.Air pressure is modulated to minimize the line spectrum. Major emphasisis aimed at achieving significant vibration energy content atfrequencies below 500 Hz.

Other prior work in relevant technology is disclosed in U.S. Pat. Nos.4,011,749; 3,686,927 and 3,710,082.

U.S. Pat. No. 4,011,749 describes a multi-degree-of-freedom shaker whoserigid test table is given time-variant displacements by a complexhydraulic actuation system with six degrees of freedom. The shaker iscontrollable at the expense of great complexity and mass.

U.S. Pat. No. 3,686,927 discloses a method for coupling selected plates,beams, or concentric cylinders with other beams or resonatingintermediate structure to effect multi-modal vibration fields for testarticles. The system described is controlled by excitation frequency andamplitude only.

U.S. Pat. No. b 3,710,082 describes a method of controlling vibrationsto a pre-determined frequency content by digitally sensing the vibrationresponse (analog plus analog to digital converter), determining thefrequency domain (Fourier transform), comparing it with a pre-determinedspectrum, combining it with (by multiplying it by) a random number (sineand cosine of four angles), transforming to a time domain (inverseFourier transform), converting to analog and subsequently exciting anelectronically driven shaker table.

SUMMARY OF THE INVENTION

A low-cost, multi-axis, quasi-random vibration system includespneumatically driven vibrators coupled to a resonating, self-attenuatingshaker structure to achieve a frequency spectrum and acceleration-levelcontrol of a broadband quasi-random vibration output in the frequencyrange, for example, from 40 Hz to 2 kHz for vibration testing ofequipment. The shaker structure consists of flexible interconnectedstructures whose primary or driving structure acts to mechanicallyprocess the vibratory input of the coupled vibrators and whosesupporting or driven structure supplies a further vibrational processingto supply the test item with multi-axis multimodal input. Automaticcontrol and pseudo-random modulation of air pressure of the pneumaticvibrators provide closed-loop broadband acceleration-spectrum controland spectrum smearing to enhance frequency content and to prevent theshaker from locking onto any particular vibration frequency, especiallya natural frequency of the shaker. Control is achieved in and aboutthree orthogonal axes simultaneously, thus affording a realisticsimulation of operational environments. The vibrators' vibrationfrequency is modulated by means of a variable-area orifice in thepneumatic line located between the air supply and the pneumaticvibrators. A pseudo-random change in the orifice area is madeautomatically, for example, every 2 to 3 seconds, by a preprogrammedmicroprocessor-controlled flow control mechanism.

More specifically, the control system controls vibration to apre-selected value by periodically sensing the vibration input to thetest item, computing the root-mean-square response, comparing it with apreselected root-mean-square value, and digitally adjusting the airsupply to the pneumatic vibrators. In addition, the predeterminedspectrum of the shaker system is controlled mechanically.

In the overall performance of the vibration scheme, the output of thepneumatic vibrators is altered through pressure modulation of thevibrators, to result in effective augmentation of the output and invibration spectrum smearing. Pressure modulation is achieved bymodulation of the area of an orifice located between the air supply andthe pneumatic vibrator drive manifold. A microprocessor is programmedwith a semi-emphirical relationship between the orifice area and thetest-item frequency response. During a test, the microprocessorperiodically varies the orifice area by means of anair-pressure-modulation flow-control mechanism and drive circuitry usinga pseudo-random number algorithm to produce a desired, e.g., uniform,distribution of values of the drive-manifold pressure.

Pressure variations produce changes in acceleration response of the testitem. The advantage of spectrum smearing and any problems resulting fromaccel-eration variation are reconciled through an automatic controlsystem. Automatic level control is based on periodic comparison of anestimate of the root-mean-square acceleration with the test-levelsetting. Acceleration feedback from the three orthogonal axes is fedthrough a low-pass filter (e.g., 2 kHz) and a sample-and-hold functionto a multiplexer, and digitized by means of an analog-to-digitalconverter. The unfiltered signals also are fed through an auxiliarymultiplexer to a pseudo-peak detector, from which is derived afast-action over-test detector function. The microprocessor subsystemperforms the balance of the data acquisition. Each of the digitalaccelerometer signals is processed to create an estimate of theroot-mean-square acceleration level. The drive pressure required for thespecified test level is adjusted periodically during a test by automaticservo adjustment of a pressure regulator through appropriate drivecircuitry.

It is, therefore, an object of the invention to produce simultaneousmulti-axial broadband quasi-random vibration.

Another object is to provide a vibration system which, in comparisonwith conventional systems, is of low cost.

Another object is to provide a microprocessor control system whichallows for fully automatic calibration, self-test and fail-safefunctions.

Another object is to provide such a vibration system which is readilyadaptable to a wide range of product sizes and shapes.

Another object is to provide a compact, self-contained system whichrequires only normal shop air and electrical power.

Another object is to provide a simple and inexpensively maintainablesystem.

Other aims and objects as well as a more complete understanding of thepresent invention will appear from the following explanation ofexemplary embodiments and the accompanying drawings thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the overall facility and modulating system thereforin block diagram;

FIG. 2 illustrates portions of the block diagram of FIG. 1 in greaterdetail;

FIGS. 3-5 depict a particular means for obtaining a variable-areaorifice by a combination of a specially shaped cam and orifice opening;

FIGS. 6-8 depict a means by which the specially shaped cam of FIGS. 3-5can be formed;

FIG. 9 depicts an alternative means for obtaining a variable-areaorifice utilizing multiple selectable flow-control orifices; and

FIG. 10 depicts a means by which the variable area orifice structure ofFIG. 9 can be constructed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a quasi-random pneumatic vibration facility andautomatic frequency modulating system is illustrated as being subdividedinto several major subsystems including a shaker table assembly 20, anair pressure and flow control mechanism 22 pneumatically coupled toshaker table assembly 20, air pressure and flow control drive circuitry24 coupled to mechanism 22 for regulating the pressure level andquantity of air delivered therethrough, a microprocessor 26 coupled toair pressure and flow control mechanism 22 for insuring that thepressure level and quantity of air delivered to shaker table assembly 20is sufficient for driving the same, a feedback and over-test protectivesubsystem 28 electrically coupled between shaker table assembly 20 andmicroprocessor 26 to insure that the microprocessor is provided with thenecessary feedback information to properly function, operator interfacesubsystem 30 for enabling an operator to establish the proper testparameters as well as to be informed thereof, and hardware protectioncircuitry 32 interposed between microprocessor 26 and drive circuitry 50for control of "ON-OFF" solenoid valves 48 in air pressure and flowcontrol mechanism 22 to protect the system from excessive vibrationallevels that might otherwise injure the device being tested. Air pressureswitches 54 and switch inputs 56 couple a drive manifold 52 in mechanism22 to microprocessor 26.

In initially describing the system operation, the output of shaker tableassembly 20 with a test item thereon is fed back through subsystem 28and compared with a preselected vibrational setting in microprocessor 26to generate an error signal which, in turn, is applied through airpressure and flow control circuitry 24 to air pressure and flow controlmechanism 22. Mechanism 22 then causes structure in shaker tableassembly 20 to vibrate a specimen in a controlled manner, and theshaker's vibratory output is fed back as described above.

Shaker table assembly 20 includes a plurality of pneumatic vibratorscollectively identified by indicium 34 which are coupled to a drivingstructure 36. The driving structure is caused to assume varying modes ofvibration based upon the excitation inputs from the vibrators inconjunction with the specifically designed physical configuration andmaterial properties. The dynamic distortions resulting from the manymodes of vibration are modified and translated to a driven structure 38by means of a visco-elastic construction 40 having, for example,resilient components 40a and damping components 40b. Driven structure38, therefore, will assume complex modes of dynamic distortion which aresuperpositions of forced and natural vibration modes of driven structure38 and of the dynamic translations imparted from driving structure 36.The vibrational output from driven structure 38 is sensed byaccelerometers 44 which are fed into feedback and overtest protectivesubsystem 28. A more complete description of the structure of shakertable assembly is described in the following patent applications, filedherewith: "Multi-Axis, Complex Mode, Pneumatically Actuated Plate/SpaceFrame Shaker for Quasi-Random Pneumatic Vibration Facility" Ser. No.897,823, "Multi-Axis, Complex Mode, Pneumatically Actuated AnnularShaker for Quasi-Random Pneumatic Vibration Facility" Ser. No. 897,824and "Nodal/Modal Control and Power Intensification Methods and Apparatusfor Vibration Testing" Ser. No. 897,822, all by Charles F. Talbott, Jr.

As described therein, the basis of the mechanical vibration deviceembodied in shaker table assembly 20 is that a structure can be excitedin many translational and rotational vibration modes, dominated bymultiples of both the excitation frequency and the natural frequency ofthe structure. The frequencies of the first few natural modes of thestructure embodied in driving structure 36 are by design not integermultiples of the primary excitation frequency obtained from pneumaticvibrators 34. The complex modal coupling between initially excitedstructure 36 and driven structure 38 results in a rich composite ofvibratory modal history arising from individual and unified structuresbehavior as modified by visco-elastic coupling 40. Test item 42 issubjected to the resulting vibration spectrum. The means by whichdriving structure 36 and driven structure 38 are coupled makes itpossible to obtain a controllable power spectrum, with specificacceleration level limits from 40 Hz to 2 kHz, which are limits ofvibration frequency for typical military specifications. As furtherdescribed in the accompanying applications, the elastomeric materials ofconstruction 40 have specifically tailored shapes and properties and areinserted with associated mechanisms between the driving and drivenstructures. The visco-elastic transmissibility and filteringcharacteristics of the chosen elastomers allow a roll-off of thevibration acceleration spectrum input to the test article at or near theupper frequency limit, regardless of the high frequencies excited indriving structure 36.

Vibrators 34 preferably comprise impacting free-piston pneumaticvibrators rather than air-cushioned free-piston pneumatic vibrators,rotary pneumatic vibrators or otherwise operated vibrators, such as byhydraulic and electro-mechanical means, but those can be used if thedesired types of dominant vibrational frequencies are obtainabletherefrom. Impact vibrators are preferred so that the sliding pistontherein impacts on at least one of the vibrator housing end surfacesafter the drive gas pressure reaches some threshold. This impact givesrise to a repeatable chain of mechanical vibratory transients that arerich in harmonic content having a very broad spectral characteristic,typically covering a range from about 50 Hz to several thousand Hertz,the upper limit depending largely on the resonant characteristic of thestructure on which the vibrator is mounted. It is preferred also to usedifferent sizes and combinations of vibrators characterized by differentrigid body fundamental frequencies for a given gas pressure inconjunction with the structural frequency response of the shakercomponents and the mass thereon. The gas pressure determines thefundamental or lowest repetition rate and the resulting impact forcelevel. Uniform vibrational energy coupling between the shaker elementsis desirable at every frequency between the lowest attainable frequencyand approximately 2 kHz. However, much of the input energy isconcentrated around multiples of the fundamental rigid body frequency ofthe free piston vibrators. This condition dictates a requirement formodulation of the drive pressure sufficient to cause an excursion of thefundamental pulse repetition frequency of 25% to 50% about the nominal.Such excursion causes a "smearing" of the frequency spectrum and assuresthat there is sufficient vibrational energy present for a predictablepercentage of the test time at every frequency.

As stated above, pneumatic vibrators 34 are actuated by air pressure andflow control mechanism 22. Specifically, a vibrator or a group ofvibrators is connected to solenoid operated air valves 48. Each airvalve 48 is maintained in an open position during operation of thesystem, and each is coupled to a drive manifold 52 for uniform supply ofair equally to all solenoid valves. If desired, a single large valve maybe attached to all vibrators, or a pneumatic valve may be utilized inplace of the solenoid to operate the air valve. In any case, solenoidvalves are electrically operated by appropriate drive circuitry 50 whichis coupled to microprocessor 26 through hardware protection circuitry32. In the event that an overtest or other damaging conditions arise,hardware protection circuitry 32 opens the circuit betweenmicro-processor 26 and drive circuitry 50 to close solenoid valve orvalves 48 and, thereby, prevent air from being supplied to vibrators 34.In such a manner, vibratory input to shaker table assembly 20 isterminated. The signal from drive circuitry 50 also causes anatmospheric vent 51, coupled to drive manifold 52, to open, thuspermitting release of pressure therefrom.

Drive manifold 52 comprises a plenum to insure a uniform flow of airequally to all vibrators. Its only constraint is that it must besufficiently small so as not to be too slow in response to changes inair pressure level and quantity of air. Air pressure switches 54 andswitch inputs 56 are coupled in series between drive manifold 52 andmicro-processor 26 and may be used for one or more purposes. They canact as limit switches to cut off the flow of air to the manifold in theevent that the air pressure drops below a preset pressure, to cut offair flow if the pressure is too high, and to assure that the pressure isadequately high in the drive manifold prior to commencement of the test.If desired, pressure transducers may be utilized instead of switches inorder to provide a means by which the exact value of pressure can bedetermined or be set for automatic operation of the system at anyacceleration level or levels.

Air to drive manifold 52 is supplied through a variable area orifice 58through a conduit 89. Its purpose is to vary the drive manifold pressurelevel with respect to time so that a greater or lesser amount of airwill be supplied to pneumatic vibrators 34 which, in turn, will thencause different levels of vibration to be exerted against drivingstructure 36. Variable-area orifice 58 is driven by a flow-controlmechanism 60 which, in turn, is driven by variable flow-control drivecircuitry 62 from microprocessor 26. The specific configuration ofvariable-area orifice 58, whether as depicted in FIGS. 3-5 or 9, willdetermine the particular structure of flow control mechanism 60. Whenconfigured as a cam and orifice mechanism as shown in FIGS. 3-5,flow-control mechanism 60 takes the form of a shaft having limitedrotational movement, such as provided by limit switches 64. For otherkinds of variable-area orifices, such as shown in FIG. 9, limit switches64 may be dispensed with.

The average pressure of air over a period of time is controlled by anair-pressure regulator 66 which, in turn, is controlled by a regulatorservo 68 and air-pressure regulator servo drive circuitry 70 operatedfrom microprocessor 26. Since regulator servo 68 is mechanical inoperation, it requires limit switches 72 to prevent overtravel of itsmechanism. The purpose of air-pressure regulator 66 is to insure thatthe proper average flow and pressure of air be supplied via conduit 87to variable-area orifice 58 over a period of time.

Air is supplied to regulator 66 from an air supply 74, and the air isfiltered through an air filter 76.

In further partial explanation of the operation of the system, beforevibrational testing of test item 42 occurs, the pressure in drivemanifold 52 is sensed through air pressure switches 54 so that airpressure regulator 66 can be set to supply that pressure and quantity ofair which is required to start pneumatic vibrators 34 when the vibrationtest begins. After the start of the test, the average of theacceleration levels for a set period of time, e.g., 21/2 minutes, issensed by accelerometers 44 to operate air pressure regulator 66.Meanwhile, the program input from microprocessor 26 to variable areaorifice 58 continues at a rapid pace, e.g., 23/4 seconds per pressurechange. The variable area orifice is varied while the averageacceleration levels are taken, in order to control the average pressureof air supplied to the pneumatic vibrators, so that the averageacceleration response (Grms) is controlled.

The electrical functions and circuitry of the system depicted in FIG. 1are shown in greater detail in FIG. 2. Air pressure and flow controlcircuitry 24, which is shown in FIG. 2, causes a stepping motor(configured as a specific embodiment of flow control mechanism 60) torotate a cam or its equivalent predictably in accordance with thecomputer program in microprocessor 26. Also, circuitry 24, through theintermediary of air-pressure regulator servo drive circuitry 70, causesair regulator 66 to change the pressure of the air supplied tovariable-area orifice 58 from a pressure switch before the test, andfrom accelerometers 44 after start of the test.

Specifically as illustrated in FIG. 2, microprocessor 26 is coupled to apair of counters 114 and 116. For each counter, whether for regulator 66or orifice 58, at zero count oscillators 118 and 120 respectivelycoupled thereto are reset and do not oscillate. Micro-processor 26through latches 122 and 124, respectively coupled to servo directioncontrols 126 and 128, latches or establishes the direction in which thepressure or the orifice area, as the case may be, will increase ordecrease. Servo direction control 126 and 128, which may comprise a pairof NAND gates, also are coupled to receive the signals transmittedbetween counters 114 and 116 and oscillators 118 and 120 and further arecoupled to drivers 134 and 136 by "up" (increase) and "down" (decrease)paths 130 and 132. Upon commencement of counting, the oscillators send apulse train over only one "up" path 130 or "down" path 132, respectivelyfor the regulator or the variable area orifice, to drivers 134 and 136in order to drive servos 68 and 60 in one of a clockwise orcounterclockwise direction.

Based upon the signal from the microprocessor and the beginning ofcounting, that is, the number of steps servos 60 or 68 are to move, theparticular oscillator begins to oscillate. The oscillation outputdecrements the counter until it again reaches zero count. At the sametime, the signal from counters 114 and 116 are fed back tomicroprocessor 26 as indicated by boxes 138 to inform the microprocessorwhether or not the oscillators are operating.

Coupled with this operation, regulator and orifice limit switches 72 and64 are actuated by the appropriate servo screw or cam to limit travelthereof. The switches are adapted to prevent signals from operating theservos beyond what is desired, as well as to so forward this informationto microprocessor 26.

The operation of feedback and overtest protective subsystem 28 is morefully described with respect to FIG. 1. Subsystem 28 receives signalsfrom accelerometers 44 and provides two functions, a first beingovertest protection and the second being notification of vibration testinformation to the microprocessor.

This latter function employs low-pass filter and sample-and-holdfunction 46, a multiplexer 140, and an analog-to-digital converter 142.Their purpose is to digitize the analog signal from the selectedaccelerometers for the purpose of determining the root-mean-squareacceleration level of the test item. For a multi-axis screeningfacility, an average of two to six accelerometer signals from at leasttwo of the three orthogonal directions is required. Multiplexer 140permits handling of signals simultaneously from more than one axis. Inoperation, microprocessor 26 through electrical connection 143 addressesthe sample-and-hold function in component 46 to have it either sample orhold the analog accelerometer signal, as well as to address mutiplexer140 to select the channel or accelerometer signal applied toanalog-to-digital converter 142. A channel selector 145 determines thenumber of accelerometer channels which microprocessor 26 is to addressto multiplexer 140.

As shown in FIG. 1, subsystem 28 has a secondary function to provide forovertest protection, utilizing auxiliary multiplexers 144, a pseudo-peakdetector 146, and an overtest protective function 148. These componentsare of conventional design. In operation, channel selector 145determines the number of accelerometer channels which auxiliarymultiplexer 144 scans so that unfiltered signals from accelerometers 44are properly fed to these components and therefrom to hardwareprotection circuitry 32. In the event that the vibrational level ofshaker table assembly 20 becomes too great, as sensed by accelerometers44, this information is processed to permit hardware protectioncircuitry 32 to interrupt the operating signal from microprocessor 26 tosolenoid valves 48, thereby to prevent further supply of air topneumatic vibrators 34.

Air pressure switches 54 are used to determine what the drive manifoldpressure is and to preset the pressure at a desired level. At least twoswitches are utilized for nominal and low pressure, respectively topreset the pressure and to turn the test off at a selected low pressureto prevent vibration below a particular switch setting. If desired, ahigh pressure switch may be used to prevent vibration above a specifiedlevel.

The operator interface subsystem, denoted generally by indicium 30, iscoupled to microprocessor 26, timing circuitry 150, and hardwareprotection circuitry 32 and embodies those functions which the operatoractuates or is displayed. A test-enable start/stop function 152 isembodied as solenoid-valve control circuitry to begin or end the test. Atest-time display 154 and a test-level display 156 both comprise numericindicators in which one shows the time and the other shows the level ofRMS acceleration during test. A test-time duration setting 158 and atest-level setting 160 comprise, for example, thumb-wheel switch arraysrespectively for setting the duration and level of the test.

Timing circuitry 150 is coupled between microprocessor 26 and hardwareprotection circuitry 32 for the purpose of enabling the operator to setthe duration of the test and to enable the control system to stop thevibration after the test time period has elapsed. It comprises aplurality of counters connected in such a manner that test durationsetting 158 presets the counter on command from the microprocessor. Theoutput from the counter is connected to display 154 to indicate the timeremaining for the test. Upon reaching zero time at the end of the testperiod, a signal is sent to hardware protection circuitry 32 whichcauses the test to stop. The same signal is also forwarded tomicroprocessor 26.

The purpose of hardware protection circuitry 32 is to interconnect thevarious failure-detect circuitry, the operator inputs, andmicroprocessor 26 (see FIG. 1). Its failure-detect control is derivedfrom the overtest protective function, the timing circuitry, and thetest enable, start and stop functions. When the test is enabled andstarted, microprocessor 26 has full control of solenoid valves 48,subject to hardware protection circuitry 32. If the overtest protectionfunction 148 detects an overtest condition, for example, themicroprocessor loses control of the servo valve. Identical results occurwhen the timing circuitry times out. The status of hardware protectioncircuitry 32 is indicated by status indicators 161.

Microprocessor 26 has several functions. It modulates the air pressure,it receives and processes vibrational signals from the accelerometers,and it performs system and self-tests. Air pressure modulation occurs byvarying the orifice area openings of orifice 58. It receives andprocesses accelerometer signals from accelerometers 44, as firstprocessed by low-pass filter and sample-and-hold function 46,multiplexer 140, and analog-to-digital converter 142. Based upon thereceipt of the accelerometer signals, the microprocessor is capable ofdetecting accelerometer anomalies. It also estimates the Grms levelwhich is displayed on test-level display 156, which is compared withinternally programmed upper and lower limits to stop vibration if theGrms exceeds the program limits, and which is used to adjust airpressure regulator 66. Its system and self-tests are to determine theoccurrence of circuitry or mechanical failure.

Such microprocessors are conventional, an 8-bit microprocessor beingsuitable for present purposes of the invention, although other sizes canbe used. Its major components include a random-access memory (RAM) and aread-only memory (ROM), with input/output latches as required. Aminicomputer or microcomputer also may be utilized.

In operation, the microprocessor is programmed to continuously changethe orifice area in variable-area orifice 58 every 1 to 2 seconds inorder to preclude shaker table assembly 20 from locking onto anyparticular vibration, especially a natural mode thereof. These changes,effected in variable area orifice 58, are pseudo-random and have anydesired distribution, e.g., uniform. As stated above, by pseudo-random,it is meant that there is a mathematical method or algorithm forselecting a sequence of numbers. This sequence is random in the sensethat it obeys certain statistical laws of randomness. By pseudo, it ismeant that the randomness is not purely random because the sequenceresults from predetermined calculations. By uniformity, it is meant thatevery drive manifold pressure is equally likely to be selected. A finitenumber of opening positions has been selected to be 128, as an example.Accordingly, the random-number algorithm in the microprocessor programis selected according to:

    N.sub.l+1 =[J+KN.sub.l ] modulo 127

where N_(l) =Random number and 0≦l≦127, and

J=a constant

K=a constant

For the cam of FIGS. 3-5, N_(l) defines the cam angular position. Forthe selectable orifice of FIG. 9, N_(l) defines the desired manifoldpressure. To obtain the desired manifold pressure, for each orificearea, there is a combination of orifices which are open. In the examplegiven, 128 positions of orifice area combination can be used, and eachposition defines a particular orifice area. Since the relationshipbetween the orifice area and the manifold pressure is known empirically(e.g., see FIG. 10), it is possible to correlate N_(l), the desiredinstantaneous maifold pressure, to the desired instantaneous combinationof open orifices.

As stated above, variable area orifice 58, its flow control mechanism 60and limit switches 64 (if needed) may be embodied as the structuredepicted in FIGS. 3-5 or FIG. 9. FIGS. 3-5 depict a cam-orificemechanism 78, comprising a cam-shaped plate 80 and an orifice opening 82in a supporting wall 84, all enclosed in a housing 85 having conduits 87and 89 respectively coupled to air pressure regulator 66 and drivemanifold 52. A drive shaft 86 is coupled to cam 80 and is driven by astepping servo motor 88. Thus, mechanism 78 defines one embodiment ofvariable-area orifice 58, while drive shaft 86 and motor 88 constituteone embodiment of flow control mechanism 60 of FIG. 1. In addition,limit switches 64 of FIG. 1 are also depicted in FIG. 5 as limitswitches 64a and 64b which define the end points of rotational travel inFIG. 5 of plate 80 in that extension 80a thereof is disposed to comeinto contact with the limit switches.

Cam-shaped plate 80 and orifice opening 82 are oriented in such a waythat edge or periphery 90 of the plate is adapted to cover or uncoverthe orifice opening to a greater or lesser extent. The shapes of bothplate periphery 90 and orifice opening 82 are configured such that, asthe angular position of the plate changes with respect to the opening,the orifice area changes according to a pre-determined relationship.During a test, the pseudo-random number algorithm in microprocessor 26causes plate 80 to be rotated every 23/4seconds with respect to orificeopening 82, either clockwise or counterclockwise, so that the orificearea defined by the combination of edge 90 and orifice opening 82randomly opens to a greater or lesser extent to permit a pseudo-randomdistribution of amounts of air to be delivered to pneumatic vibrators34.

The shape of plate edge 90 (taken in conjunction with the shape ofopening 82) is derived as follows, reference being directed to FIGS.6-8. The basic relationships are outlined in FIG. 6, which is based onthe concept that modulation of the vibration frequency is achieved bymodulation of the area of the orifice defined by the cooperation betweenopening 82 and cam edge 90. An analytical formula [1] was derivedrelating the air flow rate through the orifice to the orifice area andthe pressure in the drive manifold downstream of the orifice, asoutlined in box 92 of FIG. 6. An empirical formula [2], derived from therelationship shown in box 94, was obtained relating the flow ratethrough the pneumatic vibrators, which equals the flow rate through theorifice in steady state, to the drive manifold pressure. These formulae[1] and [2] are combined to derive an algorithm [3] (box 96), asemi-empirical formula, which defines the relationship of drive manifoldpressure to orifice area. By combining algorithm [3] with a furtherempirical relationship (box 98) of test item frequency response to drivemanifold pressure, the desired relationship (box 100) between test itemfrequency response and orifice area is derived.

Specifically, to derive empirical formula [2] (box 94 of FIG. 6), testresults from one or more particular vibrators were used to derive agraph of mass flow rate per actuator, in terms of standard cubic feetper minute (SCFM), versus drive manifold pressure (psig). For each typeof vibrator shaker table assembly, a straight line curve exists;therefore, three curves 102, 104 and 106 for different shaker tableassemblies are depicted in FIG. 7 and represent the averagesrespectively of two or more pneumatic vibrators except curve 102 whichis for a single vibrator. Empirical formula [2] is derived directly asthe mathematical representation of these straight line curves, whichwere taken from vibrators on shaker tables such as depicted in theV-shaped plate and skewed V-shaped plate constructions depicted inco-pending application Ser. No. 897,823 except curve 102 which is for asingle vibrator.

Analytical formula [1] (box 92 of FIG. 6) was calculated from therelationship of air flow rate through the orifice versus the orificearea and drive manifold pressure, as follows:

    m=31.14C.sub.D M/(1+0.2M.sup.2).sup.3 p.sub.u A            [1]

where m=flow rate in standard cubic feet per minute (SCFM),

C_(D) =function of p_(u) /p_(d) obtained from experimental data in J. A.Perry "Critical Flow Through Sharp-Edged Orifices", Trans. ASME, Vol.71, pp. 757-764, October 1949.

M=Mach number of flow through the orifice, given by ##EQU1## p_(u) 32pressure upstream of orifice 58, as controlled by pressure regulator 66,in psia

p_(d) =drive manifold pressure in psia, and

A=orifice area, in square inches.

Empirical formula [2] (box 94 of FIG. 6), as obtained from curve 104depicted in FIG. 7, is:

    m=[1.9+(0.151)(p.sub.d '-16.5)]N                           [2]

where

N=number of vibrators

p_(d) '=drive manifold pressure, in psig

Combining formulae [1] and [2], algorithm [3] (box 96 of FIG. 6) isshown as follows: ##EQU2##

The empirical relationship as shown in box 98 of FIG. 6 of the test itemfrequency response versus drive manifold pressure is obtained fromexperimental data in which a representative plot 108 thereof is depictedin FIG. 8. The empirical mathematical relationship, which is derivedfrom plot 108, is combined with algorithm [3] (box 96 of FIG. 6) toobtain the relationship of the test-item frequency response versusorifice area, as depicted in box 100 of FIG. 6, which enables one toachieve the desired goal of frequency modulation by orifice areamodulation. From this information, plate edge 90 in conjunction withorifice opening 82 is configured.

In the design with respect to the shape of cam-shaped plate 80, thedesign desired was one which would permit the drive manifold pressure tobe a linear function of the angular position of the plate. Since theorifice area is a non-linear function of the plate's angular position,the plate's shape had to be designed as illustrated. The decision toutilize the linear function was based upon the belief the the Grmsresponse was a linear function of the drive manifold pressure and thatthe programming of a linear function would be relatively simplevis-a-vis other functions. It is to be understood, however, that ifother than a linear function is desired, for whatever reason, then theshape of plate 80 and the configuration of its orifice opening 82 can bedesigned accordingly.

In a like manner, an area orifice configuration may utilize any othermechanism, such as depicted in FIG. 9, in which a plurality of solenoidoperated valves 110 permits air to flow through one or more differentlysized ports 112, 114, 116, 118 and 120. If desired, ports 112-120 may beof the same size. In any event, the combination of ports is selected toprovide a maximum total orifice area, a minimum total orifice area, andselected area sizes inbetween. Such port or valve mechanisms areexemplified in U.S. Pat. Nos. 3,726,296; 3,746,041; 3,772,877;3,785,389; and 3,875,955. By means of the above-described combinedanalytical/experimental program, a quantitative relationship betweentotal orifice area and resultant drive manifold pressure is established.This semi-empirical relationship is plotted from curve 122 taken from anillustrative design depicted in FIG. 10. During a random-vibration test,it is desired to have, for example, a uniform pseudo-random distributionof values of the drive manifold pressure, which is achieved by selectingan appropriate non-uniform pseudo-random distribution of values of thetotal orifice area from the graph in FIG. 10.

Although the invention has been described with reference to particularembodiments thereof, it should be realized that various changes andmodifications may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A vibration system comprising:means forsupporting a test item; vibrators coupled to said supporting means andoperable therewith for generating quasi-random, simultaneous multi-axisvibration in the test item; means coupled to said vibrators forautomatically causing said vibrators to vary their vibratory output andthereby to enhance the randomness in the multi-axis vibration; and meansdefining a closed loop with said vibrators for sensing and controllingthe level of the multi-axis vibration.
 2. A system according to claim 1wherein vibrator driving means are coupled to said vibrators and whereinsaid closed loop means comprises:means for detecting the vibration inputto the test item; means coupled to said detecting means for comparingsaid vibration input with a predetermined vibration input and forgenerating an error signal based on said comparison; means forprocessing the error signal; and control means coupling said processingmeans to said driving means.
 3. A system according to claim 2 furthercomprising means for causing said closed loop means means to operatepseudo-randomly and thereby to drive said driving means and saidvibrators, operable with said supporting means, to generate thequasi-random, simultaneous multi-axis vibration in the test item.
 4. Asystem according to claim 3 wherein:said detecting means includesaccelerometers; and said processing means includes means fortime-averaging the output of said accelerometers.
 5. A system accordingto claim 4 in which said vibrators comprise pneumatic vibrators and saiddriving means incudes a supply of air for said pneumatic vibrators.
 6. Asystem according to claim 5 in which said driving means further includesmeans for variably supplying the air to said pneumatic vibrators.
 7. Asystem according to claim 5 wherein said variable air supplying meansincludes means for defining at least one variable-area orifice throughwhich the air flows.
 8. A system according to claim 7 wherein saidvariable area orifice means comprises a plurality of solenoid-operatedvalves coupled together in parallel.
 9. A system according to claim 7further including:means for periodically varying the area of saidorifice means; and a pressure regulator coupled between saidvariable-area orifice means and said air supply and operatively coupledto said error processing means, for providing a proper average flow andpressure of the air to said variable-area orifice means over a period oftime which is long relative to the varying of said variable-area orificemeans.
 10. A system according to claims 2, 3, 7 or 9 furtherincluding:means defining over-test protective mechanisms coupled to saiddetecting means for sensing potentially damaging accelerations exertedon the test item; means defining a timing mechanism coupled to saidprocessing means for setting the time for operating said vibrators; andmeans defining a hardware protection mechanism coupled between saidprocessing means and said driving means for preventing operation of saidvibrators upon the occurrence of the potentially damaging accelerationsor the termination of the operating time.
 11. A system according toclaim 1 wherein said supporting means includes a vibration shaker formechanically processing vibrations.
 12. A method for quasi-randomlyvarying vibration frequencies applied to a test item comprising thesteps of superimposing a number of simultaneously produced vibrationsand applying the vibrations to the test item, wherein said superimposingstep includes the step of varying the input to means for producing thevibration frequencies rapidly with respect to averages of accelerationlevels derived from the vibrating test item.
 13. A method according toclaim 12 further including the step of utilizing air for drivingpneumatic vibrators by which the vibration frequencies are produced.